Excellent question, and a fine etymological mystery!
Astronomy did start out as an -ology like the other sciences, but it was called astrology!
Both astrology and astronomy have Greek roots, and they share the prefix astro- which means, of course, "star." The suffix -ology means "knowledge" or "science."
The suffix -onomy means "management" or "measurement." The same suffix can be found in economy (managing money), agronomy (managing land), gastronomy (the art and science of food), etc.
For much of human history, the terms "astronomy" and "astrology" have been more or less interchangable, and the astrologers were astronomers, and vice versa. However, as the two fields of study diverged, the term astrology came to be applied to the interpretation of the positions of the planets for purposes of prognostication, while the science of studying heavenly bodies was known as astronomy.
It does make a certain amount of sense. Astronomy is, strictly speaking, the measurement of the positions of the stars and planets, "ordering the stars," so to speak. In the old days, interpreting those measurements was called astrology, but nowadays we interpret our measurements not with numerology and unfounded conjecture, but with physics, and we call it astrophysics.
This page was last updated January 28, 2019.